The invention relates to a process for drying coated surfaces and to a hot-air dryer for carrying out this process in which hot air with a drying temperature T.sub.max is introduced into the dryer circulated therethrough and removed after appropriate heat transfer for example, to dry surface-coated automobile bodies.
In the field of painting automobile bodies, liquid paints are primarily used. In this case, both application techniques in which the liquid paint is sprayed in a finely atomized manner by means of spray nozzles as well as dip coating processes are used. It is common to these coating processes that in the use of a painting system, a portion of the solvent present in the paint during the coating remains in the applied paint layer. In order to avoid damage to the paint surface as a consequence of a mechanical load, these solvents must be removed or the paint layer must be hardened. For this purpose, dryers are used after the painting process.
The drying of surface-coated automobile bodies are in the most cases in the automobile industry conveyed into special drying tunnels. In this case, the coated automobile body passes through a tunnel-like furnace which is subdivided into different zones/areas in the direction of passage.
In the first area of the dryer, the coated body positioned on a transport means in the interior of the dryer is heated and a portion of the solvent of the applied surface layer is removed (radiation or heating-up zone). In this area, the applied surface layer may not be excessively mechanically loaded because it is not yet completely hardened. For this reason, the energy required to heat up the body and harden the applied surface layer is supplied contactlessly in the form of heat radiation.
One possibility to realize this in technical terms is through application of so-called radiation pockets. These radiation pockets can be heated electrically or by means of hot air streams. In both cases, it is to be taken into account that the surface temperature of the radiation pocket wall facing the object to be dried may not exceed a maximum value (T.sub.max) because the temperature in the paint coating would otherwise be too high with the result that the surface coating to be dried is damaged. Equally, the surface temperature of the radiation pocket may not fall below a minimum value (T.sub.min) because the required drying task can otherwise not be fulfilled by the dryer within a predetermined period during which the body remains in the dryer tunnel.
After the heating-up process has taken place in the radiation zone, the body to be dried passes into the second zone, the convection, circulation or holding zone. The body is held at a constant temperature level within the holding zone. During this time, the complete hardening of the paint layer takes place. In order to prevent a cooling-off of the body, heat energy in the form of a hot air stream is supplied to the body in the dryer interior.
A hot-air dryer for drying coated surfaces is known from U.S. Pat. No. 4,493,641 and comprises several area modules arranged successively in the transport direction of the body. These modules are radiation (heating-up) and convection (holding) zone modules or zone modules which can be converted by means of closable inner wall openings from convection into radiation modules. In these known drying zone modules, a partition wall is arranged in such a manner in hot-air chambers laterally surrounding the dryer or module interior such that an outer and an inner chamber is respectively formed. Both of the chambers are respectively connected at their lower end by an opening such that a deflection space is formed. The hot air supplied from above into the outer chamber flows downwardly in this, is deflected and flows upwardly in the inner chamber. In the radiation module (heating-up area), all of the hot air flows upwardly in the inner chamber up to an exhaust air channel provided above the interior space. In the convection module (holding zone), the hot air flows in the inner space at least partially upwardly and also during flow through openings in the inner wall into the dryer interior where it is drawn off at its upper end. In this known dryer, all of the supplied hot air is removed again so that a considerable hot air requirement results. Additionally, the hot air first flows through the outer chamber and then through the inner chamber so that there is already a considerable heat loss before the hot air enters the inner chamber.
The upper temperature limit and the quantity of energy to be transferred to the object to be dried determine the quantity and temperature of the hot air to be supplied to the radiation pocket or the dryer interior. This has the disadvantage for the operator of the dryer that relatively large volumes of hot air must be supplied to the dryer or removed from the dryer by a heat exchanger in the case of indirect heating or by a burner system in the case of direct heating. The possibility to operate with a smaller but more highly tempered quantity of hot air would be more favourable.